The present invention relates to means for loading wood chips into a chip digester.
Many grades of paper are manufactured from wood fibers which have been freed from the lignin which binds wood fibers together in wood. The lignin is dissolved by means of one of several chemical systems. The digestion process involves cutting the raw wood logs into wood chips which have a uniform thickness, and subjecting the wood chips to the chemical solvent under heat and pressure in a digester. A digester is a large pressure vessel in which wood chips are cooked under pressure to remove lignin, leaving so-called chemically processed wood fibers which are further processed and made into paper.
Because wood chips are typically processed in a batch process, an important aspect of the efficiency of the digestion process is minimizing the time required to load wood chips into the digester. Modern chip digesters are fabricated with air vents near the top of the digester which allow controlled removal of air from the digester as air is displaced by infeeding wood chips. Removal of air through a vent connected to the digester allows processing of the vented air to remove particles and droplets of the cooking liquor, and at the same time allows the free flow of wood chips into the digester from an overhead bin or hopper.
However older digesters were constructed without air vents and instead the air was allowed to flow upwardly through the infeeding chips and into and through the overhead chip bin. Upgrading older digester to add an air vent is an expensive and time consuming process. Digesters are pressure vessels and must conform to the ASME Pressure Vessel Code governing the construction and modification of pressure vessels. Such ASME code provisions or similar state and local codes are designed to assure safe construction, transportation, installation, and operation of boilers and pressure vessels.
Modifying an existing pressure vessel requires the entire pressure vessel to be tested in accordance with applicable code provisions. Further issues of material compatibility, weld cleanliness, weld heat effects, weld embrittlement, etc., complicate the modification of an existing digester. Significant down time required for the repair and certification under the various codes also makes modification of an existing digester expensive.
What is needed is a device or method for modifying existing digesters to vent air while loading wood chips which is less costly and results in less downtime.